In pluviometry by means of conventional transducers having some type of collecting vessel, one of the predominant measuring errors is the so-called aerodynamic error, the magnitude of which is substantially dependent upon the transducer type, wind conditions and drop size. The reason for this error is that the wind field around the transducer is distorted by the transducer itself. This distortion in turn results in a change of the paths of the water drops. For example, in a substantially cylindrical transducer, this distortion implies e.g. an increase in wind velocity and a change of wind direction above the opening of the transducer, such that the drops of water are spread and fewer drops will descend through the opening of the transducer than would have descended through the corresponding area had there been no transducer.
Many methods have been suggested in order to solve the above-mentioned problem. One method which has given improved results lies in placing the transducer in the ground with its opening at ground level. If this is correctly done, the wind field above the inlet opening will not notably deviate from what would have been the case without any transducer there. However, for practical reasons it is unsuitable to place the transducer in the ground. In fact, considerable advantages would be gained by instead placing the transducer at an optional height above ground level, e.g. at the height of a man, for this would considerably facilitate the handling of the transducer. For such measuring equipments as have been placed at a certain height above ground level, various wind shields or screens have been suggested, all aiming at reducing the magnitude of the aerodynamic error.
Also when measuring snow and other precipitation, such as hail, the same type of aerodynamically conditioned errors exist as in pluviometry. Because of the larger volume-to-weight ratio of snow as compared with water drops, the effect of a distortion of the wind field will become correspondingly greater in the case of snow. The possibility of improving the measuring results by placing the transducer in the ground is however not at hand in this case, at least for continuous measurements in wintry climates. One method that has been suggested to solve this problem is placing a collecting vessel in an extended mat of level-cut shrub vegetation. In this case, the vegetation should be so thick that the wind is almost zero therein. At the same time, it should be so thin that the snow flakes can descend through the level-cut surface and so high that a sufficiently large amount of snow can be accumulated. In practice, this method is however also unsatisfactory. Further, for snow gauges mounted above ground level, a number of different wind shields have been suggested, which have however but poorly improved the measuring results.